The invention relates to an image-forming apparatus, which is used with a plurality of different units that can be replaced with one another according to the desired toner color for printing and, more particularly, to an image-forming apparatus, in which the time of a preliminary processing of developer agitation can be varied according to the status of the apparatus.
In the prior art apparatus of this type, a plurality of units, each consisting of a photosensitive drum, a developing station and a cleaning station are provided separately for different toner colors, so that printing in various colors can be made by mounting a unit containing toner of a desired color in the apparatus body. At the time of the replacement, the replacement of consumables can be done simultaneously with the replenishment of developer or discharging of used toner.
In the unit, the developer which is contained in the developing section is prone to attenuation of its charged potential or comes to assume a semi-solidified state when the unit is left unused for long time. Further, in the case of a two-component developer, localized increase of toner concentration is liable.
Accordingly, a preliminary processing of developer agitation is executed before printing whenever the unit is replaced by opening the front door of the image forming apparatus body or the power switch is closed.
U.S. Pat. No. 4,500,195 discloses techniques concerning a replaceable unit constituting part of image formation means and also suitable alteration of the rotation of a photosensitive drum, the period of the rotation or exposure or charging condition at this time, as well as the agitation of the developer as the content of the preliminary processing noted above.
Further, U.S. Pat. No. 4,392,741 discloses a further techinque, in which time elapsed since the stopping of the rotary drum is measured, and the content of the preliminary processing is altered when and only when the measured time exceeds a predetermined period, e.g., 5 hours.
However, in the case of the former prior art example the preliminary processing noted above occurs when the power switch is re-closed after only a short period of time elapses since its opening without replacement of any unit during such time or when the door of the apparatus is opened to remove jammed sheets and is then closed again. In these situations, the preliminary processing only constitutes a waste of time. The agitation of developer as the preliminary processing is usually performed for about 60 seconds. The user, therefore, must wait for this time (e.g. about 60 seconds) until the agitation stops.
In the case of the latter prior art example, the preliminary processing is performed sufficiently only after the of 5 hours have elapsd. Therefore, in case when a replacement of a component requiring the preliminary processing is effected within 5 hours, sufficient preliminary processing is not performed in spite of the fact that the newly mounted part requires sufficient preliminary processing.